Analytic gradient for the QM/MM-Ewald method using charges derived from the electrostatic potential: Theory,...

Area of Science:

• Computational Chemistry
• Physical Chemistry
• Theoretical Chemistry

Background:

• Simulations of condensed phases require accurate treatment of boundary conditions.
• Mixed quantum mechanics/molecular mechanics (QM/MM) methods are essential for studying complex systems.
• Periodic boundary conditions are crucial for modeling bulk liquids.

Purpose of the Study:

• To implement and validate periodic boundary conditions for QM/MM simulations.
• To accurately represent periodic images of the quantum mechanical (QM) region.
• To enable stable molecular dynamics (MD) simulations of QM/MM systems.

Main Methods:

• Developed a QM/MM method incorporating periodic boundary conditions using atomic partial charges.
• Ensured variational stability by incorporating charges into the Fock matrix.
• Used least-squares fitting to derive QM atomic charges for stability across basis sets.
• Implemented analytic energy gradients for the QM/MM-Ewald method.
• Performed QM/MM simulations of a hydrated electron in water using Hartree-Fock theory plus empirical dispersion.

Main Results:

• Achieved stable molecular dynamics simulations with the QM/MM-Ewald method.
• Demonstrated the stability of the "cavity model" for hydrated electrons in liquid water.
• Observed localization of spin density within an excluded volume for at least several picoseconds.
• Validated cavity-forming pseudopotential models derived from Hartree-Fock calculations.

Conclusions:

• The implemented QM/MM-Ewald method provides stable simulations for condensed-phase systems.
• The "cavity model" of the hydrated electron is stable at room temperature.
• Results support the validity of cavity-forming pseudopotentials for aqueous electrons.
• Questioned the accuracy of non-cavity-forming pseudopotentials in representing Hartree-Fock calculations.